1. Field of the Invention
The present invention relates generally to sensing systems such as radar and sonar systems, and more particularly to methods and apparatuses for processing signals when a range is switched in a sensing system.
2. Description of the Related Art
Sensing apparatuses, such as radar and sonar systems, are used to identify objects such as moving ships, vehicles, and aircraft, stationary systems and formations, as well as altitude, speed, or acceleration of various objects, etc. A sensing apparatus typically transmits signals and receives emitted signals such as, for example, reflected or transmitted radiation, echo signals, etc. from external objects. The sensing apparatus uses these signals to obtain information about the objects associated with the signals. Information about the objects may include target images of objects, as well as spatial data related to the objects.
When a conventional radar apparatus is switched from one range (detection range) to another range to change a display range scale, target images stored on the previously used range become unusable, as these target images do not fit the new range scale. Hence, a conventional radar apparatus needs to erase all previously stored target images from an old range scale, in order to store new target images on a new range scale.
FIG. 1 is a block diagram illustrating a conventional radar apparatus, and FIG. 2 is a detailed block diagram of a signal processor in a conventional radar apparatus. The conventional radar apparatus 101 illustrated in FIG. 1 includes the following components: an antenna 1; a receiver 2; a signal processor 3; a transmitter 7; and a display 4.
The radar antenna 1 rotates at a specific speed in a horizontal plane, transmits pulses of radio waves at a specific pulse repetition rate, and receives echoes of radio waves reflected by targets. The receiver circuit 2 detects and amplifies signals received by the radar antenna 1. The signal processor 3 processes signals received from receiver 2, and sends the processed signals to display 4.
The signal processor 3 of a conventional radar apparatus 101, illustrated in FIG. 2, includes the following components: an analog-to-digital (A/D) converter 3a; a primary memory 3b; a frame memory 3c; a coordinate converter 3d; and a selector 3e. In signal processor 3, the A/D converter 3a receives an echo signal from receiver 2, and converts an analog signal obtained from receiver circuit 2 into a digital signal. The primary memory 3b stores A/D-converted data for one sweep in real time. The primary memory 3b is also used as a buffer for writing the one-sweep data into frame memory 3c in a succeeding stage, until stored data in the primary memory 3b is overwritten by new one-sweep data resulting from a next transmission. The selector 3e switches a clock input to the primary memory 3b between a write clock, which is necessary when data is written data into the primary memory 3b in real time, and a read clock, which is necessary when data is transferred to the memory.
A coordinate converter 3d generates addresses representative of pixels in the frame memory 3c, for pixels arranged in a Cartesian coordinate system successively from the center of the system outward. The coordinate converter 3d also generates a start address corresponding to coordinates of the center, based on an antenna direction (Θ) referenced to a head portion of a ship, for example. The coordinate converter 3d may also generate a location in the primary memory 3b from which a signal is read out, for example. The coordinate converter 3d is typically constructed of hardware which performs operations expressed by the following equations:X=r*cos(theta)+Xs  (1)Y=r*sin(theta)+Ys.  (2)where X and Y are coordinates of an address representative of a pixel in the frame memory, Xs and Ys are coordinates of the center address, r is the distance from the center to the pixel, and theta is the direction of a pixel for coordinate conversion.
The frame memory 3c has a storage capacity sufficient to store data received during at least one antenna rotation. A rated display controller (not shown) reads out data contents of the frame memory 3c at a high speed, and in synchronization with a scanning of display 4.
When the conventional radar apparatus 101 is switched from one range (detection range) to another to change display range scale, target images stored on the previously used range are unusable because they do not fit the new range scale (the scale associated with the new range). Hence, to store new target images on the new range scale, it is necessary to erase all the previously stored target images. As illustrated in FIG. 5, which is a flow diagram illustrating operations for a signal transferring process in the conventional radar apparatus, image data is received/created by signal processor 3 in step S111. A test is performed (S112) to determine whether a range exchange is being performed. If no range exchange is performed, image processing continues through the frame memory 3c to display 4. If, however, an exchange in range scale is performed, previous image data associated with the old range is cleared (S113).
Thus, in a conventional radar apparatus a problem occurs in that an operator of the radar system cannot recognize target ships or movements of target ships for a period of time after switching the range scale. If past target images are preserved even when the range scale has been switched, the preserved past target images might be reused when the radar apparatus is switched back to the exact original range. The preserved past target images are unusable, however, if a newly selected range differs only slightly from the original range. Hence, if the radar apparatus is switched to a range slightly different from the original range, preserved past target images from the original range are still unusable.
One way to address this problem is to prepare a plurality of frame memories, to simultaneously store target images corresponding to a plurality of ranges. This approach, however, is economically difficult to implement because of a high associated cost.
Disclosed embodiments of this application address these and other issues by implementing methods and apparatuses to process signals when a range scale is switched. In embodiments of this application, when a range scale is switched, past target images that have been stored in the former range scale are converted to fit the new range scale, so that the past target images can be continuously displayed in the new range scale. Hence, previously stored past target images in the former range scale are not erased, and can continue to provide imaging information. Embodiments of this application can be used for various sensing systems including radar and sonar systems, to process signals of various types and origins.